SEGREGATION AND HOMOGENEITY.
SECTION XHIa.—Cause of segregation.—Every liquid has a critical point in temperature below which ft may not cool without freezing. This transformation takes place by the rearrangement of the molecules into crystals, and in this rearrangement there is a tendency for each crystal-forming substance, whether an element or a compound, to separate from any substance with which it may be iniied. This tendency will result in a perfect isolation when the substances have little affinity for each other and freeze at widely different temperatures. Under these circumstances, if the temperature be slowly lowered, the more easily frozen substances will almost cbmpletely crystallize out, leaving the more fusible in a liquid state. The completeness of the separation will be lessened by a hastening of the rate of cooling, or a greater similarity between the freezing points of the mixed substances. It will also depend upon the proportion of the ingredients, for it will be more difficult for a crystal to form when its constituent molecules must find their way out of a large mass of a foreign medium, and such a crystal after so forming will be more likely to contain A certain proportion of the associated substances. Under unfavorable circumstances, as when the rate of cooling is rapid, or when the substances have nearly the same freezing temperature, or when they have an affinity for each other, the differentiation may be so much interfered with that there is no appreciable separation of the components.
All these unfavorable conditions are present in the solidification of steel.
First, the temperature of a charge, when poured from a converter or a furnace, is seldom more than 50° C. above the point of incipient congelation.